1. Field of the Invention
The present invention relates to a gas laser apparatus, and more particularly to a gas laser apparatus with which a favorable beam profile can be obtained. It also relates to a gas laser apparatus with which the flow rate of the laser gas is increased and the output energy of the laser is more stable, which makes high-repetition operation possible.
2. Description of the Related Art
With an excimer laser, whole of a rare gas is pre-ionized in a main discharge space immediately prior to discharge excitation in order to obtain a uniform glow discharge throughout the rare gas in the main discharge space.
FIG. 4 is a schematic cross section of the overall structure of a conventional excimer laser apparatus featuring pre-ionization electrodes.
As shown in FIG. 4, an excimer laser mainly comprises at least one pair of main discharge electrodes 1a and 1b which constitute a main discharge space 3 by facing each other, a laser gas G that flows from outside the main discharge space 3 into the main discharge space 3, is excited by discharge with the main discharge electrodes 1a and 1b, and flows from inside the main discharge space 3 to outside the main discharge space 3, at least one pair of pre-ionization electrodes 2a and 2b provided on the gas inflow side and the gas outflow side of the main discharge space 3 so as to sandwich the main discharge space 3 for pre-ionizing the laser gas G by directing ultraviolet light from luminescent spots Ha and Hb located around the outer periphery toward the main discharge space 3, a fan 40 for circulating the laser gas G, and a heat exchanger 41 for cooling the laser gas G flowing out of the main discharge space 3.
The pre-ionization electrode 2a on the outflow side comprises a hollow cylindrical dielectric pipe Y1, a cylindrical internal electrode F1 provided in the hollow center of the dielectric pipe Y1, and an external electrode (not shown) in contact with the outer periphery of the dielectric pipe Y1. Similarly, the pre-ionization electrode 2b on the inflow side comprises a hollow cylindrical dielectric pipe Y2 disposed at the outer-periphery, a cylindrical internal electrode F2 provided in the hollow center of the dielectric pipe Y2, and an external electrode (not shown) in contact with the outer periphery of the dielectric pipe Y2.
With the excimer laser shown in FIG. 4, the laser gas G is blown by the fan 40 in the direction L into the main discharge space 3. After this, voltage is applied between the internal electrodes F1 and F2 and the external electrodes (not shown) of the pre-ionization electrodes 2a and 2b, ultraviolet light is directed toward the main discharge space 3 from luminescent spots Ha and Hb located on the outer periphery of the dielectric pipes Y1 and Y2, and the laser gas G is pre-ionized. The pre-ionized laser gas G is excited by discharge with the main discharge electrodes 1a and 1b, and flows out of the main discharge space 3 in the direction R. As it flows in the direction R, the laser gas G is cooled by the heat exchanger 41, after which it is again blown by the fan 40 in the direction L into the main discharge space 3. Thus, with the excimer laser shown in FIG. 4, the laser gas G is circulated by the fan 40, and pulse oscillation is performed at a high-repetition frequency.
The electron density of the laser gas G is different between the situations in which the gas flows into the main discharge space 3 and in which it flows out of the main discharge space 3. This change in the electron density of the laser gas G inside the main discharge space 3 will be described through reference to FIGS. 5a, 5b, 5c, and 5d. 
FIGS. 5a, 5b, 5c, and 5d are diagrams illustrating the transition in the electron density of the laser gas G within the main discharge space 3.
First, as shown in FIG. 5a, the laser gas G1 is blown by the fan 40 in the direction L and into the main discharge space 3. Voltage is then applied between the internal electrodes F1 and F2 and the external electrodes (not shown) of the pre-ionization electrodes 2a and 2b, ultraviolet light is directed toward the main discharge space 3 from luminescent spots Ha and Hb located on the outer periphery of the dielectric pipes Y1 and Y2, and the laser gas G1 is pre-ionized to an electron density of about 108/cm3.
Next, the laser gas G1 is excited by discharge with the main discharge electrodes 1a and 1b within the main discharge space 3. The laser gas G1 is ionized when subjected to discharge excitation, so the electron density rises, resulting in a laser gas G2 with an electron density of about 1014/cm3 (FIG. 5b).
The electron density of the laser gas G2 drops after discharge excitation, resulting in a laser gas G3 with an electron density of about 1011/cm3. The electron density of the laser gas G3 is higher than that of the laser gas G1. This laser gas G3 flows out of the main discharge space 3 in the direction R (FIG. 5c).
Next, the laser gas G1 again flows in the direction L and into the main discharge space 3. Meanwhile, the laser gas G3 is blocked from flowing out by the pre-ionization electrode 2a located on the gas outflow side of the main discharge space 3, and therefore remains for a time on the gas outflow side of the main discharge space 3. The presence of the laser gas G1 and the laser gas G3, which has a higher electron density than the laser gas G1, within the main discharge space 3 changes the distribution of the electron density of the laser gas within the main discharge space 3. Voltage is then applied between the internal electrodes F1 and F2 and the external electrodes (not shown) of the pre-ionization electrodes 2a and 2b, ultraviolet light is directed toward the main discharge space 3 from luminescent spots Ha and Hb located on the outer periphery of the dielectric pipes Y1 and Y2, and the laser gases G1 and G3 are pre-ionized (FIG. 5d).
If the electron density of the laser gas here is high, the pre-ionization intensity of the laser gas will be raised, and the intensity of the laser light will also be higher.
Therefore, as shown in FIG. 5d, if the laser gas G3, whose electron density is higher than that of the laser gas G1 flowing into the main discharge space 3, remains on the gas outflow side of the main discharge space 3, the pre-ionization intensity of the laser gas G3 will be stronger than the pre-ionization intensity of the laser gas G1, so the intensity of the laser light on the gas outflow side of the main discharge space 3 will be higher than the intensity of the laser light in the center of the main discharge space 3.
FIG. 6 is a graph of beam profiles indicating the distribution of light intensity along the discharge width of the main discharge electrodes 1a and 1b. In FIG. 6, the center Xc of the discharge width corresponds to the center of the main discharge space 3, the left side of the figure corresponds to the gas inflow side of the main discharge space 3, and the right side corresponds to the gas outflow side of the main discharge space 3.
As shown in FIG. 6, the original beam profile is the beam profile Pc, in which the center Xc of the discharge width is the maximum light intensity and which is symmetrical to the left and right with respect to the center Xc of the discharge width.
In the case of FIG. 5d, however, the intensity of the laser light on the gas outflow side of the main discharge space 3 is stronger than the intensity of the laser light in the center of the main discharge space 3, so the location of the maximum light intensity of the beam profile deviates from the center Xc of the discharge width to the location XR (the right side in the figure), as shown in FIG. 6.
In other words, the beam profile in the case of FIG. 5d is the beam profile PR, which is not symmetrical to the left and right with respect to the center Xc of the discharge width.
Therefore, a problem is that the beam profile Pc, which is symmetrical to the left and right with respect to the center Xc of the discharge width, will not be obtained if the laser gas G3, whose electron density is higher than that of the laser gas G1, remains on the gas outflow side of the main discharge space 3.
In view of this, a technique in which the laser gas is not made to remain on the gas outflow side (the downstream side for the laser gas) of the main discharge space has been disclosed in the Japanese Patent No. 2,758,730 Publication.
In the Japanese Patent No. 2,758,730, a pre-ionization electrode is provided only on the gas inflow side (the upstream side for the laser gas) of the main discharge space, and no pre-ionization electrode is provided on the gas outflow side of the main discharge space.
Therefore, with the Japanese Patent No. 2,758,730, because no pre-ionization electrode is provided on the gas outflow side of the main discharge space, the laser gas does not remain on the gas outflow side of the main discharge space.
With the Japanese Patent No. 2,758,730, however, since a pre-ionization electrode is provided only on the gas inflow side of the main discharge space, the pre-ionization intensity of the laser gas on the gas inflow side of the main discharge space is stronger than the pre-ionization intensity of the laser gas present in the center of the main discharge space. In other words, the intensity of the laser light on the gas inflow side of the main discharge space is stronger than the intensity of the laser light in the center of the main discharge space.
Consequently, as shown in FIG. 6, the location of the maximum light intensity of the beam profile deviates from the center Xc of the discharge width to the location XL on the left side of the figure, resulting in a beam profile PL which is not symmetrical to the left and right with respect to the center Xc of the discharge width.
In other words, the problem is that the beam profile Pc, which is symmetrical to the left and right with respect to the center Xc of the discharge width, is not obtained.
It is an object of the present invention to obtain a more favorable beam profile.
In view of this, in order to achieve the stated object, the first present invention is a gas laser apparatus comprising at least one pair of main discharge electrodes which constitute a main discharge space by facing each other, a laser gas that flows from outside the main discharge space into the main discharge space, is excited by discharge with the main discharge electrodes, and flows from inside the main discharge space to outside the main discharge space, and at least one pair of pre-ionization electrodes provided on the gas inflow side and the gas outflow side of the main discharge space so as to sandwich the main discharge space for pre-ionizing the laser gas, wherein the beam profile indicating the distribution of light intensity along the discharge width of the main discharge electrodes changes when the pre-ionization intensity of the laser gas changes according to the amount of pre-ionization of said at least one pair of pre-ionization electrodes, characterized in that the amount of pre-ionization of the pre-ionization electrode provided on the gas outflow side of the main discharge space out of the at least one pair of pre-ionization electrodes is adjusted so as to attain the desired beam profile.
The first invention will be described through reference to FIGS. 1a, 1b and 1c, and FIG. 6.
With the first invention, of the at least one pair of pre-ionization electrodes 2a and 2b, the amount of pre-ionization of the pre-ionization electrode 2a provided on the gas outflow side of the main discharge space 3 is adjusted by varying n1, d1 and W1 so as to attain the desired beam profile. As a result, the pre-ionization intensity of the laser gas G3 remaining on the gas outflow side of the main discharge space 3 is made lower than the pre-ionization intensity of the laser gas G1 flowing into the main discharge space 3. Accordingly, the location of the maximum light intensity of the beam profile does not deviate from the center Xc of the discharge width to the location XR on the right side in FIG. 6. Also, since the pre-ionization electrodes 2a and 2b are provided on both the gas inflow side and the gas outflow side of the main discharge space 3, the pre-ionization intensity of the laser gas G1 on the gas inflow side of the main discharge space is never stronger than the pre-ionization intensity of the laser gas G1 present at the center of the main discharge space 3, as was the case with the conventional technology. In other words, the intensity of the laser light on the gas inflow side of the main discharge space 3 is never stronger than the intensity of the laser light in the center of the main discharge space 3. Accordingly, the location of the maximum light intensity of the beam profile does not deviate from the center Xc of the discharge width to the location XL on the left side in FIG. 6. As a result, the obtained beam profile Pc is symmetrical to the left and right with respect to the center Xc of the discharge width.
Therefore, a favorable beam profile can be obtained with the first invention.
Also, in order to achieve the stated object, the second invention is a gas laser apparatus comprising at least one pair of main discharge electrodes which constitute a main discharge space by facing each other, a laser gas that flows from outside the main discharge space into the main discharge space, is excited by discharge with the main discharge electrodes and thereby changes the electron density, and flows from inside the main discharge space to outside the main discharge space, and at least one pair of pre-ionization electrodes provided on the gas inflow side and the gas outflow side of the main discharge space so as to sandwich the main discharge space for pre-ionizing the laser gas, wherein the distribution of electron density of the laser gas inside the main discharge space changes and the beam profile indicating the distribution of light intensity along the discharge width of the main discharge electrodes changes when the flow rate of the laser gas within the main discharge space changes according to the locations of said at least one pair of pre-ionization electrodes, characterized in that the location of either of the at least one pair of pre-ionization electrodes is adjusted so as to attain the desired beam profile. The second invention will be described through reference to FIGS. 2a, 2b, 2c, 2d, 2e and 2f, and FIG. 6.
With the second invention, the location of either of the at least one pair of pre-ionization electrodes 2a and 2b is adjusted so as to attain the desired beam profile. When the location of the pre-ionization electrode 2a disposed on the gas outflow side of the main discharge space 3 is adjusted, it is easier for the laser gas G3, which has a higher electron density than the laser gas G1, to flow out of the main discharge space 3, and the flow rate is increased, so this laser gas G3 does not remain on the gas outflow side of the main discharge space 3. Accordingly, the location of the maximum light intensity of the beam profile does not deviate from the center Xc of the discharge width to the location XR on the right side in FIG. 6. Meanwhile, if the location of the pre-ionization electrode 2b provided on the gas inflow side of the main discharge space 3 is adjusted, the laser gas G1 will flow more readily into the main discharge space 3, the flow rates of the laser gases G1 and G3 will increase, and the laser gas G3 will not remain on the gas outflow side of the main discharge space 3. Accordingly, the location of the maximum light intensity of the beam profile does not deviate from the center Xc of the discharge width to the location XR on the right side in FIG. 6. Also, since the pre-ionization electrodes 2a and 2b are provided on both the gas inflow side and the gas outflow side of the main discharge space 3, the pre-ionization intensity of the laser gas G1 on the gas inflow side of the main discharge space 3 is never stronger than the pre-ionization intensity of the laser gas G1 present at the center of the main discharge space 3. In other words, the intensity of the laser light on the gas inflow side of the main discharge space 3 is never stronger than the intensity of the laser light in the center of the main discharge space 3. Accordingly, the location of the maximum light intensity of the beam profile does not deviate from the center Xc of the discharge width to the location XL on the left side in FIG. 6. As a result, the obtained beam profile Pc is symmetrical to the left and right with respect to the center Xc of the discharge width.
Therefore, the same effect is obtained with the second invention as with the first invention. Also, since the flow rates of the laser gases G1 and G3 are increased, there is less dispersion in the output energy of the laser, as indicated by curve A1 in FIG. 3, so the output energy of the laser is more stable and high-repetition operation is possible.